1. Field of the Invention
The invention relates to a device for detecting a driving state of a two-axle motor vehicle.
2. Description of the Related Art
The invention relates to a device for detecting a driving state of a two-axle motor vehicle, which driving state is formed from the driving situation of the motor vehicle and the state of the underlying surface, wherein output signals from vehicle on-board sensors are evaluated in order to determine the driving situation, and wherein a routine for detecting and/or estimating the coefficient of friction of the underlying surface is provided in order to determine the state of the underlying surface. Furthermore, the invention relates to a method for operating a combined vehicle brake system, in particular for motor vehicles, having hydraulically activated wheel brakes on a front axle and having electromechanically activated wheel brakes on a rear axle, wherein the vehicle wheels which are assigned to the rear axle are driven at least temporarily by an electric motor which can be operated as a generator in order to recover braking energy, and in the generator mode brings about a braking force at the vehicle wheels which are assigned to the rear axle, and wherein a pedal travel sensor detects the driver's braking request and feeds it to an open-loop and closed-loop control unit which distributes the braking force for the hydraulically activated wheel brakes, the electric mechanically activated wheel brakes and the electric motor which can be operated in the generator mode.
The purpose of such brake systems in motor vehicles is to store, as a supplement to a suitable drive, at least some of the energy in the vehicle which is recovered during braking and to re-use it to drive the vehicle. As a result, the energy requirement of the vehicle overall can be lowered, the efficiency increased and the operation therefore made more economical.
In the combined brake system which is mentioned at the beginning, a pair of hydraulic friction brakes are provided for braking the front axle wheels and a pair of electromechanically activated friction brakes are provided for braking the rear axle wheels, as are known from conventional motor vehicles, and an electric motor is used which can also be operated as a generator. At least some of the total braking force is applied by means of the generator or the electric motor which is in the generator mode. The acquired electrical energy is fed or fed back into a storage medium such as, for example, an on-board battery and is re-used to drive the motor vehicle by means of a suitable drive unit.
In order to brake such a motor vehicle which has an electric motor as a sole or additional drive which can be used to recover braking energy in the generator mode, a further braking torque is applied by the electric motor in addition to the braking torque of the hydraulically and electromechanically activated wheel brakes, which braking torque is applied by the brake system which is activated by the driver. This braking torque of the electric motor arises from the known effect in electric motors which act as a dynamo or a generator and generate electric current when they are being driven mechanically without electric current being supplied. In this context, an opposing force is produced which counteracts the mechanical drive and acts as a braking torque in the present case. The electric motor which is operated as a generator therefore acts as a brake. The total braking force of the motor vehicle which can be applied directly by the driver is therefore composed of the braking force of the hydraulically activated wheel brakes, the braking force of the electromechanically activated wheel brakes and the braking force of the electric motor which acts as a generator.
WO 99/46139 discloses a method and a device for statically or dynamically detecting setpoint values for braking forces, in which case depending on the deceleration request either the setpoint value for the electrically regenerative brake is made as large as possible or the setpoint values for the electrically regenerative brake and for the friction brake are set primarily in accordance with criteria relating to the distribution of braking force at the axles. In this context, the setpoint value for the electrically regenerative brake can be predefined in accordance with the rotational speed of the electric motor or in accordance with the charged state of the battery. In contrast, distribution of braking force as a function of the driving state is not provided.
In addition, from the theory of movement dynamics it is known that a vehicle with an overbraked rear axle has a tendency to experience movement dynamics instability.